The present invention generally relates to a phase synchronization apparatus and, in particular, relates to such an apparatus for use in a spectrometer.
In conventional spectrometers information of interest about a sample material under test is conveyed to a detector mechanism by means of a beam of light radiation. The beam of light radiation acquires the information as it passes through the sample material. Usually, for reasons well known in the art, the beam of light radiation is initially passed through the blades of a mechanical chopper and thus can be envisioned as a train of pulses. In many conventional spectrometers the train of light pulses, after passing through the sample under test, impinges upon a detector which, responsive to the light pulses, produces a pulsed electronic signal. The resultant pulsed electronic signal from the detector has, due to the pulsed nature of the beam, a periodic waveform which can be described as being equal in frequency with the train of light pulses.
In order to extract sample information from the periodic electronic signal of the sample detector it is necessary to demodulate that signal. Demodulation, in this instance, effectively involves subtracting an electronic signal representative of the beam of light radiation before it passes through the sample from the electronic signal from the detector to leave, as a remainder, only the sample information carried by the beam. The maximum sample information is extracted when the electronic signal representative of the beam of light radiation before it passes through the sample, commonly known as the reference, or demodulation, signal is exactly in phase with the periodic, or sample, signal from the detector.
Normally, a photodetector is adapted to produce an electronic signal which has the same frequency as the light pulses impinging on the photodetector. This can be accomplished by positioning the photodetector near the light chopper. Difficulties arise, however, by errors in the position of the photodetector. In one configuration, the photodetector is positioned on the sample side of the light chopper, the electronic signal thus produced will be approximately in phase with the sample signal from the sample detector. Alternatively, if the photodetector is arranged so as to receive light pulses only when the beam does not pass the chopper, then the signal from the photodetector and the signal from the sample detector will be approximately 180.degree. out-of-phase. However, the above-stated phase relationships are only valid and practical if it is assumed that the relevant optical paths are equal and that phase lead or lag in any associated electronic equipment is negligible. The phase lead or lag in the associated electronic equipment is, due to the relatively low frequency of the signals, i.e. about 15 HZ, in fact, negligible. Of course, by judiciously positioning the photodetector, some of the phase errors can be reduced. Nevertheless, the photodetector is conventionally a factory adjusted item and thus, in addition to the inherent positioning error, the degree of error varies with the frequency of the beam from the light source. As a result, the sample signal is not maximally demodulated. This results in a loss of sample information and thus the sensitivity of the entire instrument is reduced.